This invention relates to solid state circuit interrupters employing digital electronic trip units. One such solid state circuit interrupter is described within U.S. Pat. No. 4,589,052. Electronic trip units are used in such circuit interrupters and are capable of acting on a number of programmed setpoint trip parameters to interrupt the circuit being protected in a timely manner. One such electronic trip unit which allows for user interaction and which provides capability both to select trip parameters and to view the selected trip parameters is found within U.S. Pat. No. 4,672,501.
For purposes of reducing cost while maintaining accuracy and functionality of the electronic trip units, microprocessors are being employed to gather information, process the information, and provide a means of monitoring the information using associated digital circuity. The use of such microprocessors allows more protection functions to be contained in smaller-sized packages. Smaller-sized circuit interrupters because of limited space requirements do not contain the means required to enter data or to view the data being processed. The user-interfaces employed on larger-sized molded case circuit interrupters typically consist of rotatable switches with associated printed indicia to indicate the relative switch positions. The amount of area available on the front panel of such circuit interrupters is also limited and hence reduces the number of optional features requiring such switches.
U.S. Pat. No. 4,870,531 entitled "Circuit Breaker with Removable Display and Keypad", which Patent is incorporated herein for reference purposes, describes a user-interface unit that contains display capability for all possible circuit interrupter options by means of software located solely within the trip unit microprocessor while only displaying those parameters for which the electronic trip unit is configured.
U.S. Pat. No. 4,991,042 entitled "Digital Circuit Interrupter with Keypad Data Entry and Display" describes a circuit interrupter utilizing a digital keypad and display in combination with an electronic trip unit to enter and display the setpoint trip parameters. In determining the current and voltage values within the protected circuit for overcurrent and undervoltage calculations, a sampling algorithm is employed to periodically sample the current and voltage data on a continuous basis such as described within aforementioned U.S. Pat. No. 4,672,501. The sampling frequency for the sampling algorithm was provided by a fixed timer that is calculated to correspond with the 60 H.sub.z power system frequency. When the power system frequency operates at 60 H.sub.z, the sampling software is synchronized with the operating frequency in an endless loop by means of a fixed timer. In some industrial distribution circuits, the power system frequency can vary from 40 to 80 H.sub.z such that the software loop no longer corresponds to the fixed 60 H.sub.z cycle generated by the fixed timer.
Accordingly, one purpose of the invention is to provide an electronic trip circuit interrupter having digital data entry and display wherein the operating software's endless loop period within the trip unit depends on the actual operating frequency of the circuit in real time rather than a predetermined fixed time interval.